Introduction

It is well-known that wear debris generated by metal-on-metal hip replacements leads to aseptic loosening. This process starts in the local tissue where an inflammatory reaction is induced, followed by an periprosthetic osteolysis. MOM bearings generate particles as well as ions. The influence of both in human bodies is still the subject of debate. For instance hypersensitivity and high blood metal ion levels are under discussion for systemic reactions or pseudotumors around the hip replacement as a local reaction. The exact biopathologic mechanism is still unknown. The aim of this study was to investigate the impact of local injected metal ions and metal particles.

Total knee arthroplasty is a well-established treatment for degenerative joint disease, on the other hand metal ion release of cobalt or chromium and particle formation can trigger intolerance reactions. Biotribological examinations can help to assess the metal ion release in different settings. The purpose of this study was the evaluation of inter-laboratory differences in the metal ion concentration analysis. Samples were generated in a 3+1 station knee wear simulator (EndoLab GmbH, Thansau, Germany) with a medium size Columbus Knee System with or without AS multilayer coating. The wear simulation was performed under highly demanding activity (HDA) profile and samples were taken after 0.5, 5.0, 5.5. and 8.0 million cycles. The samples were blinded and sent to three different laboratories and the content of chromium, cobalt, molybdenum, nickel, and zirconium was assessed by inductively coupled plasma mass spectrometry (ICP-MS). The AS multilayer coating clearly reduced the release of chromium, cobalt and molybdenum. Mean levels were: Chromium 9329.78µg/l ± 985.44 vs 503.75µg/l ± 54.19, cobalt 10419.00µg/l ± 15.517.53 vs 2.60µg/l ± 1.35, molybdenum 2496.33µg/l ± 102.62 vs 2.46µg/l ± 2.31. Interestingly we found especially for nickel and zirconium big inter-laboratory differences in the metal assessment. There were up to 10-fold higher values in comparison of one laboratory to another. The data demonstrate that results of metal ion assessment should be evaluated by interlaboratory comparison and should be critically interpreted.

Introduction: Postoperative periprosthetic fractures are difficult to investigate clinically in scientifically and statistically valid study samples because they are uncommon. However, the combination of advanced age and poor bone quality has been associated with these fractures in conjunction with cementless hip stems. So far, these speculations have neither been supported by clinical evidence nor been investigated experimentally. The purpose of the present study was to analyze in-vitro if the treatment with cementless hip stems increases the risk of suffering a periprosthetic fracture for older patients compared to younger patients. Regarding this manner, we aimed to clarify parameters which possibly can be used preoperatively to assess age related fracture risk and whether the femoral bone quality really plays a role in fracture development.

Methods: An established biomechanical setup was used to provide an investigation on 16 femoral specimens of different age. Prior, the BMDs were measured in 5 ROIs and a cementless hip stem was implanted into each femur. The load bearing of “normal walking” was applied quasistatically under standardized conditions until the fracture occurred. The specimens were arranged by age in ascending order to divide them in the middle. A group of under septuagenarians (< 70y) (mean: 62y) and a group representing an elderly population (≥77y) (mean: 79y) resulted. Important donor data such as body height and bodyweight were considered in the statistical analysis.

Results: The elderly specimens fractured at significantly lower forces (< 70y: Fmax=5,308N; ≥77y: Fmax=2,519N; p< 0.01). Pearson’s test revealed a correlation for Fmax [N] and age (p < 0.01; r = −0.64); and for Fmax [%BW] and age (p < 0.01; r = −0.69). Fracture loads were found to correlate strongly with age (p=0.01), all used ROIs (e.g. for Ward’s triangle: p< 0.01) and BMI (p=0.04). Decreasing CCD angles were found with increasing age (p < 0.01).

Discussion: In patients with advanced age treated with cementless hip stems the risk of suffering a periprosthetic fracture is significantly higher. The identification of specific fracture development variables in geriatric populations can be extended to a preoperative check list to aid clinicians in practicing effective risk assessment. Criteria such as BMD, BMI and CCD angle should be included: A fracture risk remains in patients around 80 years of age or older, with a Ward’s triangle BMD below a value of 0.500g/cm2, or a BMI > 33kg/m2. Depending on patient activity, one single factor should not be viewed as an exclusion criterion for a cementless hip stem, whereas the cumulation of them should alert the surgeon.

Resurfacing hip implants differ in macro-and microstructure. Manufacturing related parameters like clearance or carbon content influence the wear behaviour of these metal-on-metal bearings. The aim of this study was to analyse the main macro- and micro-structural differences of commercially available resurfacing hip implants. Ten different commercially available resurfacing hip implant designs were included in this investigation:

BHR^® (Smith& Nephew/MMT) Durom® (Zimmer)

The heads and cups were measured in a coordinate measuring machine and radial clearance as well as sphericity deviation were calculated. Surface roughness measurements were carried out. The microstructures of the heads and cups were inspected using SEM and element analysis was performed using EDX to identify carbides and the alloy composition. The mean radial clearance was found to be 85.53μm. The range was from 49.47μm (DePuy, ASR®) to 120.93μm (Biomet, ReCap®). All implants showed a sphericity deviation of less than 10μm. The highest sphericity deviation was found to be 7.3μm (Corin Cormet® head), while the lowest was 0.8μm (Smith& Nephew BHR® head). On average, the heads tended to have a higher sphericity deviation (4.1μm, SD: 2.3μm) compared to the cups (2.7μm, SD: 1.4μm). SEM revealed that most manufacturers use a high carbon alloy casting manufacturing process combined with heat treatment after casting (Corin Cormet® and Wright Conserve®: head and cup; DePuy ASR®: cup; Eska BS®: head). Commercially available resurfacing hip implants differ in design and manufacturing parameters, including macro- and microstructure, which are critical in achieving low wear and ion release. This study was designed to aid in the understanding of clinical observations. Also, specific information is now available for surgeons choosing an implant designs.